Seemingly fed up with the authorities' inability or unwillingness to address their concerns, mainland petitioners have resorted to a new and unlikely channel - the Obama White House.

In recent days, tens of thousands of mainlanders have signed a petition to the US government's public website, We The People, calling on the Obama administration to intervene in the two-decade-old poisoning case and help bring the suspect, believed to be living in the US, to justice.

Nineteen years ago, Tsinghua University student Zhu Ling was left paralysed and intellectually handicapped by a suspected dose of highly toxic thallium. The main suspect was her roommate, Sun Wei, also known as Jasmine Sun.

The petition, which had gathered more than 144,000 signatures as of last night, sought help from Washington to convince Chinese police to reopen the case and to investigate an alleged sham marriage that petitioners allege allowed Sun to settle in the US more than a decade ago.

Analysts say the petition to a foreign government to intervene in a domestic affair underscores the level of public distrust in the country's judiciary and police, and frustration with its opaque political system.

The petition also unleashed a torrent of discussion in mainland media last week about Zhu's plight and the social and political implications of her case.

Zhu returned to the public spotlight in April when, in a similar but unconnected case, Shanghai police arrested a Fudan University postgraduate medical student in connection with the death of his roommate, who was also poisoned.

That was in stark contrast to the way police handled Zhu's case in 1994 - they abruptly closed it after a few months, leaving the public to speculate about the cause of her poisoning and whether there had been interference in the investigation as the suspect was from a well-connected family. To this day, the police are criticised for refusing to share information with Zhu's parents.

Mainland media have been largely sympathetic to public calls for greater transparency in the case. Last week, Xinhua raised seven questions about the matter, including what evidence had been suppressed, why police had refused to disclose their findings and whether there had been outside pressure on the investigation.

"The failure of police to answer these questions only raises more doubts, which is unfair to the victim, Zhu Ling, and the suspect, Sun," it added.

The public appeal for US government intervention prompted a rare response on Wednesday from Beijing police, who denied that officials had meddled in the investigation. Instead, they blamed delays in reports from other parties, an absence of key evidence and a lack of modern-day technology such as surveillance cameras for the failure to solve the case.

People's Daily said online on Thursday that while the police response was long overdue, it at least showed a positive attitude.

"The breadth of speculation over the Zhu Ling case is the result of authorities' ambiguous responses and withholding information from her family," it said.

Citing criticism of the latest police response from internet commentators, Global Times, a tabloid affiliated with People's Daily, argued that police might have done right by playing down the case 19 years ago because it would have been controversial regardless of how they responded.

Petitioning the We the People site has quickly become popular among mainland internet users.

The Youth Times newspaper said on Wednesday that the internet had empowered the public in ways that went beyond the nation's borders, raising pressure for greater transparency and the rule of law.

The paper said: "After all, it's not a glorious thing that Beijing police have failed to solve the case after 19 years, particularly when they have offered no credible reason for putting the case on hold for so long."

Yes it is never a glorious thing for police anywhere to shelve a case as unsolved, but it is at least better to put a possibly innocent person behind the bar. Unless fresh evidence turn up, forcing the police to reopen the case merely shows popular excess.